Objectives: (a) The analysis of the genetic regulation of rates of synthesis and degradation of mammalian hepatic phenylalanine hydroxylase utilizing the following systems: (1) Tissue culture studies; hepatic explant culture, hepatic single cell culture and minimal deviation hepatoma cells; (2) Animal studies; gene expression of foetal and neonatal rat phenylalanine hydroxylase; and (3) the spontaneous mutant mouse designated dilute lethal d1/d1. (b) The analysis of the regulation of phenylalanine hydroxylase activity by small molecules such as substrate, substrate analogues, product and cofactor. (c) Gene dosage effects in normal, heterozygote, abnormal homozygote and hyperphenylalaninemics. The parameters to be determined are the activation rate, the pseudo-first order rate constant, the Michaelis constant and Vmax for phenylalanine and tetrahydropteridine. The long range goal of this project is the resolution of genetic and metabolic control mechanisms of phenylalanine hydroxylase and delineation of the molecular parameters of the type of enzymatic defect in different subpopulations of PKU with the hope of defining new avenues of therapeutic approach for each type.